Cassette-Based Equipment: The Computed Radiography

CASSETTE-BASED EQUIPMENT:

THE COMPUTED RADIOGRAPHY

CASSETTE, IMAGING PLATE,

AND READER AREEJ ALOUFI

CR SYSTEM COMPONENTS

PSP – photostimulable phosphor plate

REPLACES FILM IN THE CASSETTE

CASSETTES

phosphor plates

ID STATION

IMAGE PREVIEW (QC) STATION

DIGITIZER

VIEWING STATION

COMPUTED RADIOGRAPHY EQUIPMENT

Cassette

Imaging plate

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CASSETTE

Looks like a film/screen cassette

Durable, lightweight plastic

Backed by aluminum

No intensifying screens

Antistatic material

4

Cassette

Aluminum

Backing

Antistatic

Material

IMAGING PLATE

Construction

Image recorded on a thin sheet of plastic known as the

imaging plate

Consists of several layers:


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IMAGING PLATE

Protective layer: A very thin, tough, clear plastic for

protection of the phosphor layer

Phosphor, or active, layer

A layer of photostimulable phosphor that “traps” electrons

during exposure

Usually made of phosphors from the barium fluorohalide

May also contain a dye that differentially absorbs the stimulating

laser light to prevent as much spread as possible




IMAGING PLATE

Reflective layer: A layer that sends light in a forward

direction when released in the cassette reader

May be black to reduce the spread of stimulating light and the

escape of emitted light

Some detail lost in this process

Conductive layer: A layer of material that will absorb and

reduce static electricity




IMAGING PLATE

Color layer: Newer plates may contain a color layer, located

between the active layer and the support that absorbs the

stimulating light but reflects emitted light.

Support layer: A semirigid material that gives the imaging

sheet some strength.

Backing layer: A soft polymer that protects the back of the

cassette.




CASSETTE AND IMAGING PLATE

Cassette contains a barcode label on the cassette or on the imaging plate viewed through a window in the cassette.

Label enables technologist to match the image information with the patient-identifying barcode on the exam request.




CR

PSP in cassette

Digital image

Scanned & read- CR reader

COMPUTER

Image stored on computer

Viewed on a Monitor

Hard copy (film) can be made with laser printer

GENERAL OVERVIEW

CR

PSP cassette exposed by

conventional X-ray equipment.

Latent image generated as a matrix of trapped electrons in the plate.

CR Phosphor Plates

ABSORPTION EMISSION

X-RAY

LIGHT

LASER STIMULATION

ELECTRON

TRAP

ELECTRON

TRAP

CR – PSP PLATE

Stimulated by a RED LIGHT

Energy is RELEASED in a form of BLUE light

LIGHT captured by PMT –

changed to a digiial signal

HOW CR WORKS

Released light is captured by a PMT (photo

multiplier tube)

This light is sent as a digital signal to the computer

The intensity (brightness) of the light – correlates

to the density on the image

Statistical plots of the frequency of occurrence of

each pixel's value

ERASING PLATE

After image is recorded

Plate is erased with high intensity white light

and re-used

THE READER

With CR systems, no chemical

processor or darkroom is

necessary.

Cassette is fed into a reader:

Removes the imaging plate

Scans it with a laser, releasing the

stored electrons




THE LASER

Light Amplification by Stimulated Emission of

Radiation.

Laser creates and amplifies a narrow, intense beam of

coherent light.

Laser requires a constant power source to prevent output

fluctuations.

Laser beam passes through beam-shaping optics to an

optical mirror, which directs the laser beam to the surface

of the imaging plate.




• Reader scans the plate with red light in a zigzag, or

raster, pattern.

• Laser gives energy to the trapped electrons.

• Red laser light is emitted at approximately 2 eV,

which is necessary to energize the trapped electrons.

• Extra energy allows the trapped electrons to escape

the active layer, where they emit visible blue light at

an energy of 3 eV as they relax into lower energy

levels.

USING THE LASER TO READ

THE IMAGING PLATE

The light collection optics direct the released phosphor energy to an optical filter and then to the photodetector.

Although there will be variances between manufacturers, the typical throughput is 50 cassettes per hour.

Some manufacturers claim up to 150 cassettes per hour, but based on average hospital department workflow,

50 cassettes per hour is much

more realistic.




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In the process of digitizing the light signal, each phosphor storage center is scanned .

The light is directed to the photomultiplier/charged

coupled device that converts the light to an electronic signal.

That electronic signal is digitized by an analog to digital converter (ADC).

The ADC assigns each picture element or pixel a value

that corresponds to a level of brightness. The entire image is divided into matrix of pixels based

on the brightness of each pixel. Each square is called a pixel or picture element.

DIGITIZING THE SIGNAL


The typical number of pixels in a matrix range from about 512 × 512 to 1024 × 1024 and can be as large as 2500 × 2500. As the number of pixels in a matrix increase for the same field

of view, the smaller the pixels have to be to fit into that area; the smaller the pixels, the greater the spatial resolution.

The image is digitized by position (spatial location) and by intensity (gray level).

Each pixel contains bits of information, and the gray level is determined by how many photons struck the imaging plate in that particular location. Bit depth: The number of bits per pixel.





The bit depth will be a factor in determining the quality of the image.

A bit value of 21 or one (1) represents a black and white image.

The larger the bit depth, the more shades of gray possible.

The more shades of gray, the more detail the image can display.

Each pixel can have a gray level between 2 (21) and 4096 (212).




SPATIAL RESOLUTION

Spatial Resolution

Refers to the amount of detail present in any image.

Phosphor layer thickness and pixel size determines resolution in CR.

The thinner the phosphor layer is, the higher resolution.




SPEED

CR systems use the terms speed class or speed class

of operation.

The speed class is a reflection of the amount of light

(photostimulable luminescence [PSL]) emitted by the plate

during the reading process and is related to the level of

exposure received by the receptor.

The level of exposure received by the imaging plate indicates

the speed class of operation.




ERASING THE IMAGE

The process of reading the image returns most of electrons to a lower energy state.

Reading effectively removes the image from the plate.

Imaging plates are extremely sensitive to scatter radiation.

Plates should be erased to prevent a buildup of background signal.

Plates should be run at least once a week under an erase cycle to remove background radiation and scatter.

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PREPROCESSING, PROCESSING, AND

FORWARDING THE IMAGE

Once the imaging plate has been read, the signal is sent to the computer.

Image is preprocessed.

A monitor displays the image so that the technologist can do the following:

Review the image

Manipulate it if necessary (postprocessing)

Send it to the quality control station and ultimately to the picture archiving and communications system (PACS)

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SUMMARY

The cassette-based imaging system consists of a

specially designed cassette made of durable,

lightweight plastic.

The imaging plate is multilayered with protective,

phosphor, reflective, conductive, color, support, and

backing layers.

Barcodes are used to identify the cassette or imaging

plate and exam request to link the imaging plate with

the patient exam.

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SUMMARY

Barium fluorohalide crystals in the imaging plate

release light energy that is stored in the conductive

layer.

The imaging plate reader uses a laser to scan the

imaging plate, releasing the energy stored in the

conductive layer as blue light.

A photomultiplier amplifies the light and sends it to a

signal digitizer.

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SUMMARY

The digitizer assigns a numeric value to each pixel in a

matrix according to the brightness of the light and its

position.

Spatial resolution of the digital image is determined by

the thickness of the phosphor layer and the number of

pixels, which also effects resolution and sampling

frequency. Cassette-based spatial resolution is

approximately 2.55 lp/mm to 5 lp/mm (lower than 10

lp/mm of conventional radiography).

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SUMMARY

Because so many more densities are recorded in CR (wide dynamic range), images have increased contrast resolution but not increased spatial resolution.

Because energy stored in the imaging plate is lost over time, imaging plates should be read as quickly as possible to avoid image information loss.

Imaging plates are erased by exposing them to bright light such as fluorescent light.

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SUMMARY

The image is sent to the quality control station (which may

be the same as the technologist workstation), where it is

analyzed and sent to PACS for long-term storage.




Areej Aloufi

Documents

Cassette-Based Equipment: The Computed Radiography